1. Field of the Invention
The present disclosure relates generally to dead reckoning systems and more particularly to a dead reckoning altimeter apparatus using a speedometer and a forward-looking accelerometer for measuring changes in altitude. The present disclosure also relates particularly to an inclinometer apparatus using a speedometer and a forward-looking accelerometer for measuring inclination angle.
2. Description of the Background Art
Dead reckoning (DR) is the process of estimating one's current position based upon a previously determined position and advancing that position based upon measured speed, direction and/or acceleration. The DR begins with an initial known position, or fix. The fix can be determined using ranging, triangulation or map matching. It is common to use radio signals for ranging from the global navigation satellite system (GNSS) for establishing an initial position fix from which to start dead reckoning.
Dead reckoning speed can be measured by many methods. Before modern instrumentation, DR speed was determined aboard ship by throwing a wood float, called a log, overboard and counting the knots on a line tied to the float that passed a sailor's hand in a sandglass measured time as the ship moved forward through the water. More modern ships use engine rpm, automatic logs for measuring water speed, or bottom looking Doppler sonar. Road vehicles typically measure speed by measuring revolution rates of their wheels. Road vehicles can also use engine rpm and Doppler sonar or radar for speed measurement. The horizontal direction can be measured with a magnetic or flux gate compass. Dead reckoning direction can also be determined by integrating the rate of change of angles sensed by an angular rate sensor. An angular rate sensor is sometimes referred to as a gyro. Inertial systems that integrate directional linear accelerations can be used for dead reckoning, especially for aircraft.
Even with the advancement of the convenience and accuracy of the global navigation satellite system (GNSS), there continues to be a need for dead reckoning for cases when continuous GNSS fixes cannot be obtained or are noisy. Further, global navigation satellite system positioning tends to be less accurate and noisier for altitude and vertical heading angles than for horizontal positions and horizontal heading angles.